Conventional drill bits, as used for example in the drilling of bores in the oil and gas exploration and production industry, feature a number of toothed roller cones. In use, weight is applied to the rotating bit, and the cones roll over the circular face forming the end of the bore. The cutting of the rock is achieved by a number of mechanisms, including a crushing action, the crushed rock then being removed from the cutting face by the action of drilling fluid exiting the drill bit via appropriately directed jetting nozzles. In addition, by varying the orientation of the rotational axes of the cones, it is possible to provide a scraping or scouring action as the cones rotate.
A more recent development, allied to advancements in drilling and materials technology, has lead to the development of drill bits featuring fixed cutting faces provided with relatively hard materials, typically polycrystalline diamond compact (PDC).
Traditionally, drilling of the deep bores required in the oil and gas industry relied solely upon applied weight, which weight may be applied from surface, or achieved simply by the mass of the drill string, drill collars and other tools and devices above the bit. However, in recent years drilling has commenced in areas featuring particularly hard rock, in which drilling using conventional methods is still possible, but relatively slow. In attempts to overcome this problem, there have been various proposals for percussive or hammer drill bits; while air-powered hammer drills are in common use in, for example, the construction trades and in mining, drilling of deep bores with non-compressible drilling fluid poses many different and varied problems. One difficulty has been in reliably achieving the necessary impact forces at the bit. The limited diameter available downhole limits the size of the hammer tool, resulting in excessive loading and wear on the tool in order to achieve the required impact force. Another difficulty that has been experienced in percussive bits is the tendency for the bits to “lose gauge”, that is the outer edges of the bits wear prematurely, with the result that the bit drills a smaller diameter bore than intended, or must be replaced at frequent intervals.
SU 1730420 A1 (Leningrad Scientific-Research & Design Institute “Gipronikel”) discloses a combined percussive rotary drilling tool. In addition to a conventional roller-cutter, the tool features a central percussive bit which cooperates with a piston actuated by a supply of compressed air. Clearly, such an arrangement would not be suitable in downhole operations where there is no supply of compressed air available; conventionally, drilling “mud” is supplied to the drill bit, which liquid could not be utilised to operate a piston as disclosed in this document.
U.S. Pat. No. 3,807,512 discloses a percussion-rotary drilling mechanism with a rotary drill bit and a percussion drill bit. A mud drive turbine is utilised to generate reciprocal motion of the percussion drill bit via an arcuate cam and cam follower drive mechanism or a rotatable eccentrically weighted wheel drive mechanism.